The present invention relates to a novel polyelectrolytic gel.
In secondary lithium batteries, condensers, capacitors, sensors or like devices using an electrolyte, the electrolyte should be tightly enclosed therein to prevent a liquid leakage or a fire due to short circuits, and it should be firmly encased therein to avoid an accident due to an impact. In view of this necessity, lightweight devices have been difficult to produce. For overcoming this drawback, it is desired to solidify an electrolyte.
For solidification of electrolytes, solid electrolytes and polyelectrolytic gels have been proposed which are applicable to electrochemical devices such as secondary batteries, condensers, capacitors, sensors or the like. For example, the proposals include solid electrolytes which comprise a mixture of an electroconductive powder with polyacrylonitrile or an acrylonitrile/methyl (meth)acrylate non-crosslinked copolymer (Electrochimica. Act., Vol.37, No.10, 1851-1854, 1992, JP-A-4-306560 and JP-A-7-45271); and polyelectrolytic gels produced by using a crosslinked polymer formed from polyethylene glycol diacrylate, polyethylene glycol dimethacrylate or the like (JP-A-62-285954 and JP-A-6-68906).
However, these solid electrolytes and electrolytic gels have drawbacks that they are low in heat resistance and that when heated to 60xc2x0 C. or higher, the gel becomes structurally collapsed and unsuitable for use at a high temperature.
For improvement of heat resistance, JP-A-10-144137 proposed a polyelectrolytic gel prepared by cooling a polymer solution to a low temperature below 0xc2x0 C., e.g. xe2x88x9220xc2x0 C. to cause gelation, the polymer solution containing a non-crosslinked copolymer of acrylonitrile and methyl (meth)acrylate or vinyl acetate dissolved in a nonaqueous solvent containing an electrolyte. However, the obtained gel is instable in electroconductivity, particularly ion conductivity. When the gel is used for secondary lithium-ion batteries or electrolytic condensers, the devices not only become instable in ion conductivity immediately after production, but also increase the resistance due to repeated charge and discharge, resulting in rapid reduction of ion conductivity.
Since acrylonitrile polymers or the like used for conventional electrolytic gels or solid electrolytes are mostly produced by copolymerizing monomers having carboxyl group or sulfonic acid group, the gels and solids are likely to contain a large amount of alkali metal cations or alkaline earth metal cations or anions of sulfuric acid, nitric acid or like acids as counter ions.
For example, when an electrolytic gel using an acrylonitrile polymer contains 500 ppm or more of alkali metal ions or alkaline earth metal ions other than lithium ions, lithium ions are low in mobility, i.e. ion conductivity and also become lower in ion conductivity by repeated charge and discharge, so that the resulting secondary lithium-ion battery is given a short service life and is unsuitable for use.
Stated more specifically, cations or anions larger in ionic radius than lithium ions not only reduce the mobility of lithium ions in the polyelectrolytic gel but also degrade the properties of electrode materials and electrolytes by repeated charge and discharge, thereby gradually lowering the electroconductivity and shortening the service life of batteries or condensers.
It is an object of the present invention to provide a novel polyelectrolytic gel which is excellent in heat resistance and durability and is superior in electroconductivity, particularly ion conductivity.
Other objects and features of the present invention will become more apparent from the following description.
The present inventor conducted extensive research and found that the foregoing object can be achieved by a polyelectrolytic gel wherein a polymer component is swollen with a nonaqueous solvent containing an electrolyte dissolved therein, the polymer component having nitrogen-containing cationic functional group and possessing a crosslinking structure. The present invention was completed based on this novel finding.
According to the present invention, there are provided the following polyelectrolytic gels.
1. A polyelectrolytic gel comprising a polymer component and a nonaqueous solvent, characterized in that the polymer component is a crosslinked polymer having nitrogen-containing cationic functional group or a mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of nitrogen-containing cationic functional group, the polymer component being swollen with the nonaqueous solvent containing an electrolyte dissolved therein.
2. The polyelectrolytic gel according to item 1, wherein the nitrogen-containing cationic functional group is at least one species selected from the class consisting of free primary amino group, secondary amino group or tertiary amino group, primary ammonium base, secondary ammonium base, tertiary ammonium base or quaternary ammonium base which ammonium bases have formed a salt with carboxy anion, and quaternary ammonium base having formed a salt with hydroxy anion.
3. The polyelectrolytic gel according to item 1, wherein the polymer component is a crosslinked polymer having nitrogen-containing cationic functional group.
4. The polyelectrolytic gel according to item 3, wherein the crosslinked polymer having nitrogen-containing cationic functional group is obtained by polymerizing and crosslinking 100 parts by weight of an unsaturated monomer free of nitrogen-containing cationic functional group, 1 to 100 parts by weight of an unsaturated monomer having nitrogen-containing cationic functional group and 1 to 50 parts by weight of a crosslinkable monomer having at least two reactive functional groups per molecule.
5. The polyelectrolytic gel according to item 4, wherein the unsaturated monomer free of nitrogen-containing cationic functional group is acrylonitrile.
6. The polyelectrolytic gel according to item 4, wherein the crosslinkable monomer has at least two reactive functional groups selected from the class consisting of hydroxyl, carboxyl, glycidyl, vinyl, isocyanate and methylol.
7. The polyelectrolytic gel according to item 3, wherein the crosslinked polymer having nitrogen-containing cationic functional group is obtained by polymerizing 100 parts by weight of an unsaturated monomer free of nitrogen-containing cationic functional group and 1 to 100 parts by weight of an unsaturated monomer having nitrogen-containing cationic functional group to give a non-crosslinked polymer, and crosslinking the resulting non-crosslinked polymer with 1 to 200 parts by weight of a crosslinking agent.
8. The polyelectrolytic gel according to item 7, wherein the non-crosslinked polymer has at least one species of reactive functional group selected from the class consisting of hydroxyl, carboxyl, glycidyl, vinyl, isocyanate and methylol, and wherein the crosslinking agent is a compound having at least one species of reactive functional group selected from the class consisting of hydroxyl, carboxyl, glycidyl, vinyl, isocyanate and methylol, the functional group of the crosslinking agent being complementarily reactive to the reactive functional group of the non-crosslinked polymer and being at least two in number per molecule.
9. The polyelectrolytic gel according to item 7, wherein the unsaturated monomer free of nitrogen-containing cationic functional group comprises acrylonitrile and the unsaturated monomer having at least one species of reactive functional group selected from the class consisting of hydroxyl, carboxyl, glycidyl and methylol.
10. The polyelectrolytic gel according to item 1, wherein the polymer component is a mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of nitrogen-containing cationic functional group.
11. The polyelectrolytic gel according to item 10, wherein the non-crosslinked polymer having nitrogen-containing cationic functional group is obtained by polymerizing 100 parts by weight of an unsaturated monomer free of nitrogen-containing cationic functional group and 1 to 100 parts by weight of an unsaturated monomer having nitrogen-containing cationic functional group.
12. The polyelectrolytic gel according to item 11, wherein the unsaturated monomer free of nitrogen-containing cationic functional group is acrylonitrile.
13. The polyelectrolytic gel according to item 10, wherein the mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of nitrogen-containing cationic functional group is obtained by polymerizing and crosslinking 10 to 200 parts by weight of a polymerizable compound having at least two crosslinkable functional groups per molecule in the presence of 100 parts by weight of a non-crosslinked polymer having nitrogen-containing cationic functional group.
14. The polyelectrolytic gel according to item 13, wherein the polymerizable compound is a compound having at least two crosslinkable functional groups per molecule, the crosslinkable functional groups being at least one species selected from the class consisting of vinyl, glycidyl, isocyanate, hydroxyl and methylol.
15. The polyelectrolytic gel according to item 1, wherein the cation species of the electrolyte in the gel is lithium ion and the total amount of alkali metal ions other than lithium ions and alkaline earth metal ions is 500 ppm or less based on the polymer component.
16. The polyelectrolytic gel according to item 1, wherein the electrolyte is at least one species selected from the class consisting of LiClO4, LiBF4, LiPF6, LiAsF6, LiSCN, Lil, LiBr, Li2B10Cl10, CF3SO3Li and LiC(SO2CF3)3.
17. The polyelectrolytic gel according to item 1, wherein the amount of the nonaqueous solvent is 100 to 5,000 parts by weight per 100 parts by weight of the polymer component.
18. The polyelectrolytic gel according to item 1, wherein the nonaqueous solvent is an aprotic solvent.
19. The polyelectrolytic gel according to item 18, wherein the nonaqueous solvent has a boiling point of 90xc2x0 C. or higher.
20. The polyelectrolytic gel according to item 1, wherein when heated to 80xc2x0 C., the gel is neither dissolved nor brings about a phase separation into liquid phase and solid phase.
21. The polyelectrolytic gel according to item 1, wherein the gel is in the form of a membrane having a thickness of 1 to 500 xcexcm.
The polyelectrolytic gel of the present invention is resistant to heat at 80xc2x0 C. or higher, non-degradable in properties by repeated charge and discharge, namely highly durable and is excellent in electroconductivity, particularly in ion conductivity.
The polyelectrolytic gel of the present invention comprises a polymer component and a nonaqueous solvent, the polymer component having nitrogen-containing cationic functional group and being swollen by absorbing a large amount of the nonaqueous solvent containing an electrolyte dissolved therein, and the polymer component possessing a crosslinked structure which is a three-dimensionally reticulated structure insoluble in the nonaqueous solvent. The amounts of the polymer component and the nonaqueous solvent to be used are about 100 to about 5,000 parts by weight of the latter per 100 parts by weight of the former. The electrolyte exists as dissolved in the nonaqueous solvent.
The polymer component in the gel of the present invention essentially has nitrogen-containing cationic functional group. The nitrogen-containing cationic functional group is a nitrogen-containing substituent which can become a cation or has become a cation, on reaction with an acid.
Examples of the nitrogen-containing cationic functional group are free amino group, ammonium bases having formed a salt with carboxy anion, ammonium base having formed a salt with hydroxy anion, etc. The nitrogen atom of the nitrogen-containing cationic functional group may be one constituting the nitrogen-containing heterocycle.
The polyelectrolytic gel using the polymer component containing such nitrogen-containing cationic functional group has a polarizing current value higher by 5 times than conventional polyelectrolytic gels,.as measured by cyclic voltammetry.
This feature is presumably derived from the following. For example, anions such as BF4xe2x88x92, PF6xe2x88x92, AsF6xe2x88x92, Ixe2x88x92, Brxe2x88x92, CIO4xe2x88x92, SCNxe2x88x92, CF3SO3xe2x88x92, C(SO2CF3)3xe2x88x92 or the like larger in radius than-lithium ions are present as counter ions in a lithium compound often used as an electrolyte, and are scavenged by the nitrogen-containing cationic functional group in the polymer component, whereby the mobility, namely transport number, of lithium ions is enhanced.
When the nitrogen-containing cationic functional group has formed a salt with a strong acid such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, sulfonic acid or the like, the ability of increasing the transport number of lithium ions is lowered, and the ion conductivity of the polyelectrolytic gel is increased in a reduced degree but is higher than conventional gels free of said functional group. The reason for this effect remains to be clarified but it is presumably because there may exist a hollow space in the gel wherein the lithium ions can move relatively easily due to the presence of bulky, large side chain bonded to the polymer molecule.
When the nitrogen-containing cationic functional group in the polymer has formed a salt with a carboxylic acid compound, the obtained gel shows a slightly lower ion conductivity than when a free type is used but a higher ion conductivity than an electrolytic gel using a polymer free of said functional group.
Since the electrolyte dissolved in the nonaqueous solvent is usually a strong acid salt, the anion of the carboxylic acid compound having formed a salt with the cationic functional group is replaced with the anion of the electrolyte, so that the lithium ions can become easily movable and the ion conductivity is presumably unhindered.
Consequently preferred nitrogen-containing cationic functional groups for use herein include free primary amino group, free secondary amino group, free tertiary amino group, primary ammonium base having carboxy anion as a counter anion, secondary ammonium base having carboxy anion as a counter anion, tertiary ammonium base having carboxy anion as a counter anion, quaternary ammonium base having carboxy anion as a counter anion, and quaternary ammonium base having hydroxy anion as a counter anion.
The nitrogen-containing cationic functional group need not be a free amino group or ammonium base having carboxy anion or hydroxy anion as a counter anion in the preparation of the polyelectrolytic gel of the present invention. For example, if the nitrogen-containing cationic functional group of the unsaturated monomer is free and instable and it is difficult to polymerize the monomer in this state, the polymer obtained by polymerization of the monomer having formed a salt with an acid can be converted to a polymer having free amino group or ammonium base having said counter anion by washing with a solution of lithium hydroxide, sodium hydroxide, potassium hydroxide or the like or by using an anion exchange resin.
It is essential in the present invention that the polymer component constituting the gel of the invention have a crosslinked structure to retain the form of the gel. Accordingly the polymer component should be a crosslinked polymer having nitrogen-containing cationic functional group or a mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of cationic functional group.
For ease of production and quality of product, the crosslinked polymer having nitrogen-containing cationic functional group is preferably prepared by polymerizing an unsaturated monomer having nitrogen-containing cationic functional group and crosslinking the obtained polymer compound.
Preferred crosslinked polymers having nitrogen-containing cationic functional group include, stated more specifically:
(i) a polymer prepared by polymerizing and crosslinking 100 parts by weight of an unsaturated monomer free of nitrogen-containing cationic functional group, 1 to 100 parts by weight of an unsaturated monomer having nitrogen-containing cationic functional group, and 1 to 50 parts by weight of a crosslinkable monomer having at least 2 reactive functional groups per molecule; and
(ii) a polymer prepared by polymerizing.100 parts by weight of an unsaturated monomer free of nitrogen-containing cationic functional group and I to 100 parts by weight of an unsaturated monomer having nitrogen-containing cationic functional group to give a non-crosslinked polymer, and crosslinking the resulting non-crosslinked polymer with 1 to 200 parts by weight of a crosslinking agent having at least 2 reactive functional groups per molecule.
Other crosslinked polymers having nitrogen-containing cationic functional group include:
(iii) a polymer obtained by crosslinking a polymer compound free of nitrogen-containing cationic functional group with a crosslinking agent having nitrogen-containing cationic functional group. Examples of such crosslinked polymer include a crosslinked polymer prepared by crosslinking with a polyamine compound a polymer compound free of nitrogen-containing cationic functional group and having an epoxy group.
The unsaturated monomer having nitrogen-containing cationic functional group is one having functional groups such as primary amino group, secondary amino group, tertiary amino group, primary ammonium base, secondary ammonium base, tertiary ammonium base, quaternary ammonium base, nitrogen-containing heterocyclic residue, residue of heterocyclic salt which has become a cation or the like. Among them, preferred are primary to tertiary amine compounds having free (without forming a salt with an acid) primary to tertiary amino groups and primary to quaternary ammonium compounds having carboxy anion as a counter anion, and quaternary ammonium compounds having hydroxy anion as a counter anion.
The number of nitrogen-containing cationic functional groups in the unsaturated monomer having nitrogen-containing cationic functional group is not limited to one group per molecule, and may be more than one. The kind of nitrogen-containing cationic functional group is not limited to one species but may be a mixture of primary to quaternary functional groups.
Preferred examples of monomers having a single nitrogen-containing cationic functional group are allylamine, methallylamine, N-ethylamino acrylate, N-propylamino methacrylate, N-methylaminoethyl acrylate, N-diethylaminoethyl acrylate, N-diethylaminoethyl methacrylate, N-dimethylaminopropyl acrylamide, N-diethylaminoethyl methacrylamide, N-trimethylaminoethyl acrylate, N-triethylaminoethyl methacrylate, etc.
Preferred examples of monomers having two or more nitrogen-containing cationic functional groups are a composite of primary amine and secondary amine such as aminoethyl methallylamine, a composite of secondary amine and tertiary amine such as N-dimethylaminopropyl allylamine, (N-diethylaminoethyl)-Nxe2x80x2-ethylaminoethyl methacrylate and the like.
Examples of unsaturated monomers free of nitrogen-containing cationic functional group are not limited insofar as they are copolymerizable with an unsaturated monomer having nitrogen-containing cationic functional group, and include acrylonitrile, methacrylonitrile, vinyl compounds, acrylate compounds, etc. Among them, acrylonitrile is preferred.
Examples of vinyl compounds are vinyl acetate, vinyl propionate, vinyl butyrate, styrene, vinyl pyridine, N-vinyl pyrrolidone, etc. Examples of acrylate compounds are methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate and like alkyl esters of (meth)acrylic acid; hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate and like hydroxyalkyl esters of (meth)acrylic acid; diethylene glycol acrylate, diethylene glycol methacrylate, triethylene glycol acrylate, triethylene glycol methacrylate, polyethylene glycol acrylate, polyethylene glycol methacrylate, polypropylene glycol acrylate, polypropylene glycol methacrylate and like compounds optionally having a terminal hydroxyl group substituted with alkyl to convert to methoxy, ethoxy, buthoxy or like group.
Among the unsaturated monomers free of nitrogen-containing cationic functional group, it is preferred to use reactive monomers containing hydroxyl, carboxyl, isocyanate, methylol, glycidyl or the like to give reactivity with crosslinkable monomers or crosslinking agents.
Examples of such reactive monomers are allyl alcohol, methallyl alcohol, N-methylol acrylamide, N-methylol methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, methylenebis acrylamide, methylenebis methacrylamide, glycerol monomethacrylate, acrylic acid, methacrylic acid, itaconic acid, maleic acid, glycidyl acrylate, glycidyl methacrylate, etc.
The amount of the unsaturated monomer having nitrogen-containing cationic functional group is usually 1 to 100 parts by weight, preferably 5 to 100 parts by weight, more preferably 10 to 100 parts by weight, per 100 parts by weight of the unsaturated monomer free of nitrogen-containing cationic functional group. Less than 1 part of the unsaturated monomer having nitrogen-containing cationic functional group results in less effect of increasing the transport number of lithium ions, whereas more than 100 parts by weight thereof can not achieve a higher effect.
The crosslinkable monomer having at least two reactive functional groups per molecule is used simultaneously with the unsaturated monomer having nitrogen-containing cationic functional group and the unsaturated monomer free of nitrogen-containing cationic functional group to undergo polymerization and crosslinking reactions in a single step for the purpose of introducing a crosslinked structure for increase of heat resistance of polyelectrolytic gel, giving a crosslinked polymer having nitrogen-containing cationic functional group.
On the other hand, the crosslinking agent containing at least two reactive functional groups per molecule is used as follows. The unsaturated monomer having nitrogen-containing cationic functional group and the unsaturated monomer free of nitrogen-containing cationic functional group are polymerized to give a non-crosslinked polymer having nitrogen-containing cationic functional group, and the crosslinking agent is added to the obtained non-crosslinked polymer to crosslink the same. In other words, the crosslinking agent is used in producing the crosslinked polymer having nitrogen-containing cationic functional group by a two-step process.
When the gel is prepared by the single step, the amount of such crosslinkable monomer is 1 to 50 parts by weight, preferably 2 to 40 parts by weight, per 100 parts by weight of the unsaturated monomer free of nitrogen-containing cationic functional group and per 1 to 100 parts by weight of the unsaturated monomer having nitrogen-containing cationic functional group. Less than 1 part of the crosslinkable monomer or crosslinking agent lessens the degree of crosslinking, making it difficult to solidify the gel, and gives a low heat resistance to the gel. Hence it is undesirable. On the other hand, more than 50 parts by weight of the crosslinkable monomer or crosslinking agent excessively increases the degree of crosslinking, giving a hard and brittle polymer and a gel which is likely to develop cracks. Namely it results in production of a polymer unsuitable for use and is undesirable.
When the gel is prepared by the two-step process, the amount of such crosslinking agent is 1 to 200 parts by weight per 100 parts by weight of the unsaturated monomer free of nitrogen-containing cationic functional group and per 1 to 100 parts by weight of the unsaturated monomer having nitrogen-containing cationic functional group.
The crosslinkable monomer or crosslinking agent for use in the present invention are compounds having at least two reactive functional groups per molecule.
Examples of compounds having two bonded reactive functional groups such as hydroxyl, carboxyl, glycidyl, vinyl, isocyanate, methylol or the like are ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, ethylene glycol diacrylate, polyethylene glycol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, divinylbenzene, polyethylene glycol diisocyanate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate, 1,9-nonanediol dimethacrylate, glycidyl acrylate, glycidyl methacrylate, N-methylol acrylamide, N-methylol methacrylamide, hydroxyethyl methacrylate, methylenebis acrylamide, methylenebis methacrylamide, 1,4-butanediol, 1,6-hexanediol, 1,9-nonanediol, polyethylene glycol, polypropylene glycol, tetramethylene glycol, adipic acid, sebacic acid, dimer acid, etc.
Examples of compounds having three bonded reactive functional groups are trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, trimethylpropane triglycidyl ether, trimethylolpropane, citric acid, etc.
Examples of compounds having four or more bonded reactive functional groups are pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, pentaerythritol, etc.
When the crosslinked polymer having nitrogen-containing cationic functional group is prepared by separately carrying out polymerizing and crosslinking reactions in two steps, the non-crosslinked polymer has at least one species of reactive functional group selected from the class consisting of hydroxyl, carboxyl, glycidyl, vinyl, isocyanate and methylol, and the crosslinking agent is preferably a compound having at least one species of reactive functional group selected from the class consisting of hydroxyl, carboxyl, glycidyl, vinyl, isocyanate and methylol, the functional group of the agent being at least two in number per molecule and being complementarily reactive to the reactive functional group of the non-crosslinked polymer.
The polymer component constituting the gel of the present invention may be a mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of nitrogen-containing cationic functional group.
When the polymer component is said mixture, a preferred polymer component is obtained by polymerizing and crosslinking 10 to 200 parts by weight of a polymerizable compound having at least 2 crosslinkable functional groups per molecule in the presence of 100 parts by weight of a non-crosslinked polymer having nitrogen-containing cationic functional group.
The non-crosslinked polymer having nitrogen-containing cationic functional group is a homopolymer of an unsaturated monomer having nitrogen-containing cationic functional group or a copolymer of said unsaturated monomer and the unsaturated monomer free of nitrogen-containing dationic functional group. A preferred non-crosslinked polymer is one obtainable by polymerizing 100 parts by weight of an unsaturated monomer free of nitrogen-containing cationic functional group and 1 to 100 parts by weight of an unsaturated monomer having nitrogen-containing cationic functional group. The foregoing non-crosslinked polymer has a weight average molecular weight of preferably about 5,000 to about 1 million, more preferably about 10,000 to about 500,000. When the molecular weight is less than 5,000, the gel may be too soft to retain the form at a high temperature. In the case of more than 1 million, the solution is given too high a viscosity in production, becoming difficult to handle. Hence the molecular weight outside said range is undesirable.
Desirable as said polymerizable compound having at least two crosslinkable functional groups per molecule is said crosslinkable monomer having at least two functional groups per molecule. The crosslinkable monomers may be used either alone or in combination with a monomer which is copolymerizable therewith and is free of nitrogen-containing cationic functional group.
When 10 to 200 parts by weight of the polymerizable compound having at least 2 crosslinkable functional groups per molecule is polymerized and crosslinked in the presence of 100 parts by weight of the non-crosslinked polymer having nitrogen-containing cationic functional group, there is obtained a mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of cationic functional group which is a polymer or copolymer of the polymerizable compound. The mixture presumably comprises the non-crosslinked polymer and the crosslinked polymer as tangled with each other at a molecule order without being chemically bonded. When the amount of the polymerizable compound is less than 10 parts by weight per 100 parts by weight of the non-crosslinked polymer, the mixture is difficult to gel sufficiently, whereas more than 200 parts by weight forms a highly crosslinked structure, giving a gel with low flexibility and making it difficult to obtain a gel suitable for use. Hence it is undesirable.
In preparing the polyelectrolytic gel of the invention, it is possible to add, when so required, other polymer compound in addition to the polymer component comprising a crosslinked polymer having nitrogen-containing cationic functional group or a mixture of a non-crosslinked polymer having nitrogen-containing cationic functional group and a crosslinked polymer free of nitrogen-containing cationic functional group. Examples of other polymer compounds are polyethylene oxide, polypropylene oxide, polybutylene oxide and copolymers of these oxides like polyalkylene glycol which has two terminal hydroxyl groups substituted with alkyl groups. Among them, dimethoxypolyethylene glycol with a molecular weight of about 500 to about 3,000 provides a gel with high ion conductivity and superior polarizing current value and is desirable.
Suitable electrolytes in the gel of the invention include, for example, LiClO4, LiBF4, LiPF6, LiAsF6, LiI, LiBr, LiSCN, Li2B10Cl10 and like inorganic lithium compounds and CF3SO3Li, LiC(SO2CF3)3 and like organic lithium compounds. However, useful electrolytes are not limited to these examples insofar as they can be dissolved in the nonaqueous solvent.
The concentration of the electrolyte in the nonaqueous solvent can be determined depending on the kind of electrolytes and the desired level of electroconductivity, but is about 0.1 to about 3 mole/l, preferably about 0.3 to about 2 mole/l.
The nonaqueous solvent dissolving the electrolyte for use herein is not aqueous and is capable of dissolving a non-crosslinked polymer having nitrogen-containing cationic functional group. Preferred solvents are those having a water content of 1% by weight or less. Aprotic solvents are desirable to use.
Recommendable nonaqueous solvents include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methylethyl carbonate and like carbonate solvents, ethylene glycol, propylene glycol, methyl cellosolve, ethyl cellosolve and like ether solvents, xcex3-butyrolactone, sulforan, adiponitrile, glutaronitrile, N-methyl pyrrolidone, trimethyl phosphate, etc.
Preferred solvents are those having a high boiling point, preferably a boiling point of higher than 90xc2x0 C., insofar as the non-crosslinked polymer having nitrogen-containing cationic functional group has a high solubility. The solvents with a boiling point of lower than 90xc2x0 C. are readily volatile and are likely to cause problems due to its high vapor pressure. Hence they are undesirable.
These solvents can be used either alone or in combination. For example, to enhance the electrochemical properties at low temperatures, a mixture of two, three or more solvents is preferable to use in view of the viscosity and boiling point of the solvent.
The amount of the nonaqueous solvent in the polyelectrolytic gel is preferably about 100 to about 5,000 parts by weight, more preferably about 500 to about 5,000 parts by weight, per 100 parts by weight of the polymer component.
Less than 100 parts by weight fails to give a gel having high flexibility and excellent processability. On the other hand, more than 5,000 parts by weight gives a soft gel, or leaves a non-solid product or a viscous solution or brings about a phase separation into solid gel and nonaqueous solvent, resulting in difficulty in achieving the contemplated object.
The polyelectrolytic gel of the invention can be suitably prepared as by the following processes.
(1) A process to be conducted comprises the steps of dissolving an unsaturated monomer having nitrogen-containing cationic functional group, an unsaturated monomer free of nitrogen-containing cationic functional group, a crosslinkable monomer and an electrolyte in a nonaqueous solvent, and heating the solution or irradiating the solution with an activation energy rays such as ultraviolet rays, electron rays or the like to conduct polymerization and crosslinking, giving a polyelectrolytic gel.
(2) A non-crosslinked polymer is prepared by polymerizing an unsaturated monomer having nitrogen-containing cationic functional group and optionally an unsaturated monomer free of nitrogen-containing cationic functional group. A process to be carried out comprises the steps of dissolving the non-crosslinked polymer, a crosslinking agent and an electrolyte in a nonaqueous solvent, and heating the solution or irradiating the solution with an activation energy rays such as ultraviolet rays, electron rays or the like to crosslink the polymer, giving a polyelectrolytic gel. In this process, optionally the non-crosslinked polymer having nitrogen-containing cationic functional group can be used in combination with a non-crosslinked polymer free of nitrogen-containing cationic functional group.
(3) A process to be performed comprises the steps of dissolving the non-crosslinked polymer prepared in the same manner as in the process (2), a polymerizable compound having crosslinkable functional groups and an electrolyte in a nonaqueous solvent, heating the solution or irradiating the solution with an activation energy rays such as ultraviolet rays, electron rays or the like to polymerize and crosslink the polymerizable compound, giving a polyelectrolytic gel. In this process, optionally the non-crosslinked polymer having nitrogen-containing cationic functional group can be used in combination with a non-crosslinked polymer free of nitrogen-containing cationic functional group. When required, the polymerizable compound having crosslinkable functional groups can be used in combination with other polymerizable compound(s).
(4) A process to be conducted comprises the steps of causing the previously prepared crosslinked polymer having nitrogen-containing cationic functional group to swell with a nonaqueous solvent containing an electrolyte dissolved therein, giving a polyelectrolytic gel.
According to the processes (1) to (3), a gel of crosslinked polymer containing the electrolyte and nonaqueous solvent can be formed from a homogeneous solution of the monomer or polymer with the progress of crosslinking while retaining the homogeneous state. Consequently the processes provides a gel comprising the crosslinked polymer, electrolyte and nonaqueous solvent much more homogeneouly mixed with each other than the process (4) in which the previously prepared crosslinked polymer is caused to swell with the solvent to give a gel. When the gel obtained by the processes (1) to (3) is used for secondary lithium batteries, condensers, capacitors or other electrochemical devices, it is easy to manufacture a stable device of high performance. Accordingly any of the processes (1) to (3) is preferable as a process for preparing the gel of the present invention.
In the case of the process (1), it may be difficult to remove the unreacted monomer from the gel because the polymerization and crosslinking reactions are carried out in a single step. In this case, the gel may contain a large amount of impurities. Especially when the unreacted monomer of low boiling point such as acrylonitrile is left in the gel, the unreacted monomer may generate bubbles due to heat in the gel, making it difficult to produce a gel of high performance. If an attempt is made to remove the unreacted monomer from the gelled polyelectrolyte under reduced pressure, the unreacted monomer may be gasified and bubbles are increased in the gel, probably failing to give a dense gel.
In the case of processes (2) and (3), the non-crosslinked polymer to be used is purified before dissolving the same in the nonaqueous solvent, or after preparation of a polymer in the nonaqueous solvent, a solution of the polymer is usable from which the unreacted monomer of low boiling point has been removed under reduced pressure. The obtained gel contains no or little impurities, and bubbles are unlikely to be generated in the gel. In other words, these processes are recommendable.
In order to overcome the foregoing problem that secondary lithium batteries are instable in ion conductivity, it is desirable in the polyelectrolytic gel of the present invention that the cation species of the electrolyte in the gel be lithium ion and that the total amount of alkali metal ions other than lithium ions and alkaline earth metal ions existing as counter ions of anionic group in the polymer component of the gel be preferably 500 ppm or less, more preferably 100 ppm or less, most preferably 50 ppm or less, based on the polymer component.
To reduce the content of ionic metal impurities, it is important what polymerization catalyst is selected for solution polymerization of the monomer mixture in preparing the gel of the invention, as well as to avoid as much as possible the use of a monomer containing an anionic group having formed a salt with alkali metals other than lithium or alkaline earth metals.
Depending on the kind of polymerization catalysts, the catalyst may be partly taken in the polymer as an ionic terminal group of the polymer. For example, in redox polymerization using sodium sulfite and sodium persulfate, sodium sulfonate derived from the polymerization initiation radical is formed as a polymer terminal group so that a large amount of sodium ions exists in the polymer.
To eliminate the alkali metal ions or alkaline earth metal ions from the polymer, a method of washing with an aqueous solution of strong acid is available, but is not efficient from the viewpoint of commercial manufacture. Consequently such polymerization catalyst should be used in a quantity as small as possible or the use thereof should be avoided to meet the object of reducing the quantity of ionic impurities.
From the above viewpoints, it is recommendable to conduct solution polymerization using a polymerization catalyst free of alkali metals such as benzoyl peroxide, azobisisobutyronitrile, benzyldimethyl ketal or the like or using means such as irradiation with electron rays, UV rays or other rays.
The amount of the polymerization catalyst to be used is varied depending on the kind of catalysts or the desired level of molecular weight and is indeterminable. It is usually in the range of about 0.01 to about 5% by weight based on the total amount of monomers.
The gel of the present invention has a gel structure having intermolecular strong chemical bonds which structure is attributed to the crossliked structure of the polymer component. The gel of the invention is radically different from physical gels free of crosslinked structure. Thus the gel of the invention can be heated without loss of gel form and is excellent in heat stability.
The gel of the invention is stable in the form at a high temperature. For example, when heated even to 80xc2x0 C. for 1 hour, the gel neither brings about a phase separation into liquid phase of nonaqueous solvent and solid phase of solid gel nor is lowered in transparency. In short, the gel of the invention is outstanding in heat stability.
In secondary batteries or capacitors which require a high output, a laminate of cells is used wherein intermediate cells may be increasingly heated to an unexpectedly high temperature by repeated charge and discharge. To maintain safety, the heat resistance of the gel in individual cells is very important. The gel of the invention can be suitably used for this purpose.
In the above processes, a mixture of monomers can be subjected to solution polymerization in a nonaqueous solvent containing an electrolyte dissolved therein or can be gelled by crosslinking, preferably using means such as irradiation with electron rays, UV rays or heating. Polymerizations available in the above processes include, for example, addition polymerization, ring opening polymerization, condensation polymerization and the like.
The time for polymerization or gelation is widely varied depending on the temperature, polymerization methods, crosslinking methods and other factors. For example, xcex3-ray irradiation takes less than 1 minute, UV irradiation needs about 1 to about 30 minutes, and heating requires about 10 to about 300 minutes.
When a solution of monomers and other starting materials is charged or poured into a transparent container or a film seal pack, electron ray irradiation and UV irradiation can be efficiently used. In the case of using an opaque container or metal container, heating or the like is utilized. In any case, although a polymerization catalyst is advantageously used for efficient progress of polymerization, a catalyst free of alkali metals other than lithium and alkaline earth metals is suitable to use as described hereinbefore.
The shape of the gel according to the present invention is not limited. Usually the gel can be formed in a shape such as sheets, membranes, spheres, cubes, rectangular parallelepipeds, cylinders, combinations of these shapes, etc. in conducting crosslinking reaction during the production operation. The obtained gel can be given the desired shape by suitable fabrication such as shearing, cutting, crushing or the like.
The gel of the invention to be used as an electrolyte, e.g., for secondary battery cells, is preferably in the shape of a membrane having a thickness of 1 to 500 xcexcm.
The present invention will be described in detail with reference to the following representative Examples and Comparative Examples. However, the present invention is not limited to the Examples. The parts and percentages in the Examples and Comparative Examples are all by weight unless otherwise specified.
The evaluation methods used in the Examples and Comparative Examples are as follows.
Evaluation of Electrochemical Properties
A measurement cell produced from a gel sample was attached to an alternating impedance measurement device (product of Solatorn Co., Ltd., xe2x80x9c1286+1250xe2x80x9d). The alternating impedance at 100 KHz to 1 Hz at 20xc2x0 C. was measured, and the impedance values at a measurement frequency of 100 KHz and 100 Hz were taken as a bulk resistance value and an interfacial resistance value, respectively. The ion conductivity was calculated from the bulk resistance value and the thickness and area of the cell.
The measurement of alternating impedance was continued for 24 hours, and then the measurement cell was connected to a device for the evaluation of electrochemical properties (product of Solatorn Co., Ltd., xe2x80x9cS1-1280Bxe2x80x9d) to carry out polarization electrolysis by a cyclic voltammetry at a turn over voltage of xc2x10.5 V and a scan rate of 10 mV/s at 20xc2x0 C. After three cycles of voltage scanning, a polarizing current value was measured at +0.5 V and the obtained value was taken as a CV polarizing current value.
After the measurement of polarizing current value, an alternating impedance was measured again to give an interfacial resistance value 24 hours after cell assemblage.
From these measured results, a ratio of interfacial resistance value immediately after commencement of measurement to the value 24 hours thereafter was calculated to obtain an increase ratio of interfacial resistance.
The measurement cells of Examples 1 to 34 and Comparative Examples 1 to 4 were assembled as follows. First, a gel membrane having a thickness of about 120 xcexcm was cut to a square shape 2xc3x972 cm. The sample was held at both sides between twin metal lithium foil pieces (0.5 mm in thickness and 2xc3x972 cm) and was placed into a pack of laminated aluminum film lined with a polyethylene film. The pack was connected to a lead wire and was heat-sealed under reduced pressure. A cell was assembled in a globe box with the air replaced with argon gas.
The measurement cells of other Examples are described in each example.
Evaluation of Heat Resistance
A gel sample was placed into a sample bottle and the bottle was heated in a hot-air circulating thermostatic container at 80xc2x0 C. for 1 hour. Then the bottle was taken out from the container to observe the condition of the gel. The condition of the gel was evaluated and rated according to the following criteria.
A: The gel showed no change of form before and after heating, and was excellent in heat resistance.
B: After heating, the gel was partly dissolved or caused a slight phase separation, and was a little low in heat resistance.
C: After heating, more than 50% of the gel was dissolved or phase-separated and was very low in heat resistance.
The gel samples of Examples 1, 8 to 10, 17 to 34 and Comparative Examples 1 to 3 were prepared as follows. A sample bottle 24 mm in inside diameter and 40 mm in height was charged with 10 ml of a solution containing acrylonitrile, a monomer having nitrogen-containing cationic functional group, a crosslinkable monomer, a nonaqueous solvent containing an electrolyte dissolved therein, a polymerization catalyst and the like. Then the mouth of the bottle was covered with aluminum foil, and the solution was polymerized and crosslinked, giving a gel sample.
The gel samples of Examples 42 to 84 were prepared as follows. A gel membrane 200 xcexcm in thickness was cut to a size 1xc3x972 cm and was introduced into a sample bottle 24 mm in inside diameter and 40 mm in height. The mouth of the bottle was covered with aluminum foil to provide samples.
Evaluation of Flexibility
Gel samples were bent at an angle of 90 degrees at room temperature to observe whether cracks were created at the bent portion. The flexibility was evaluated according to the following criteria.
A: No crack developed, leaving no trace of fold and thus the gel was highly flexible.
B: Although no crack developed, a slight trace of fold was left. The gel was slightly poor in flexibility.
C: Cracks developed and the gel lacked flexibility.
The gel samples of Examples 1 to 41 and Comparative Examples 1 to 4 were prepared as follows. A solution containing acrylonitrile, a monomer having nitrogen-containing cationic functional group, a crosslinkable monomer, a nonaqueous solvent containing an electrolyte dissolved therein, a polymerization catalyst and the like was placed into a seal pack of polyethylene film (100 xcexcm in thickness) having inserted therein a silicone rubber sheet 1 mm in thickness as a spacer. In the pack held between twin glass plates, the solution was polymerized and crosslinked, giving a gel sample.
Gels in the form of a membrane 200 xcexcm in thickness were used as gel samples in Examples 42 to 84.
Measurement of Sodium Ion Concentration
The concentration of sodium ions in the gel or the polymer was measured by atomic absorption method using a measurement device (xe2x80x9cAA-6500xe2x80x9d, product of Shimadzu Mfg. Co., Ltd.). The limit of detected value was 10 ppm. When the concentration of sodium ions in the polymer constituting the gel was measured, the gel was heated under reduced pressure to remove the nonaqueous solvent and was dried under reduced pressure for 24 hours before measurment. Thereafter the measurement was carried out.
(I) Examples using, as the polymer component, a crosslinked polymer having nitrogen-containing cationic functional group which was prepared by polymerizing and crosslinking an unsaturated monomer free of nitrogen-containing cationic functional group, an unsaturated monomer having nitrogen-containing cationic functional group and a crosslinkable monomer.